The research study focuses on the vertical distribution profiles of traffic-generated PM2.5/NO2 concentration in several typical naturally-ventilated high-rise public residential buildings of point and slab block designs, at different parts of Singapore. A total of six buildings were selected for the study and these buildings are located in close proximity to expressways that had high traffic volume.
Experimental results showed that PM2.5 mass/number concentration and NO2 concentration was highest at the mid floor of the building as compared to those measured at high and low floors. PM2.5/NO2 emitted from hot vehicle exhaust rise due to buoyancy and blown towards the building by the upstream wind. PM2.5/NO2 motion in the upward and sideways direction is further assisted by the wind turbulence additionally amplified by the fast moving vehicles along the expressway. Upon reaching the trees, some of the PM2.5/NO2 are being intercepted by tree leaves. Most of the PM2.5/NO2 flow over the top of the trees and keep moving upwards and towards the building carried by airflow streamlines. This could explain the reason for the PM2.5 mass/number concentration and NO2 concentration being the highest at the mid floor of the buildings as compared to those measured at high and low floors. Although the lower floors were closest to traffic emissions, the PM2.5 mass/number concentration and NO2 concentration was lower there than that at the mid floors, which is due to the buoyancy rise at the source point, interception of PM2.5/NO2 by tree leaves or the vortices at the wake region of the trees diluting the traffic-polluted air behind the trees or all three. The high floors had the least PM2.5 mass/number concentration and NO2 concentration due to dilution following pronounced mixing of traffic-polluted air with ambient air. The only difference between the point block and slab block configurations is that at corresponding floors, the PM2.5 mass/number concentration /NO2 concentration for slab block is much higher than that of point block under similar traffic and meteorological conditions. This is attributable to the slab block configuration which tends to slow down wind speed. The vertical distribution profile of PM2.5 mass/number concentration and NO2 concentration in this study differs from the vertical distribution profile of several studies which found that PM2.5 mass/number concentration and NO2 concentration usually decreased with increasing height. However, in previous studies, there were no trees in between the motorways and buildings.
The health risk model show for both the blocks, residents at the mid floors of the buildings have the highest health risk for all age categories: infants, children (1yr), children (8 - 10 yr) and adults in the mid floor compared to the high (lowest) and low floors (second highest) due to PM2.5/NO2 inhalation. This was expected since the highest concentration of to PM2.5/NO2 concentration occurred at the mid floors of the buildings. For both the blocks, new born babies, one year old children, and adults had similar potential health risk while teenage children (8 - 10yr) had the lowest potential health risk due to PM2.5 inhalation. For NO2 inhalation, one year old children in both the blocks suffer from the highest health risk followed by 8 - 10 year old children. New born infants had the least health risk. The health risk model results also showed that for the point block, NO2 and the combined effect of PM2.5 and NO2 is about 2.3 and 3.3 times more risky than PM2.5 respectively and for the slab block, NO2 and the combined effect of PM2.5 and NO2 is about 2.1 and 3.2 times more risky than PM2.5 respectively. Living in a slab block is about 1.37 times more risky due to PM2.5 and about 1.27 times more risky due to NO2 in contracting a respiratory disease compared to living in a point block